Australian captain Michael Clarke was on track for a Test double century when rain stopped play on day two of the third and deciding Test with South Africa in Cape Town.

Clarke was the star on day two, reaching 161 not out at stumps - his eighth-highest Test innings - with Ryan Harris on four not out as Australia ended the day on 7 for 494.

Clarke had resumed on 92, and he moved quickly to 99 before becoming becalmed on the verge of his century.

Kyle Abbott produced a tight spell of bowling to concede one run off six overs, and in the end Clarke faced 24 balls on 99 before finally breaking the drought with a cover drive off Philander for a boundary to reach his ton.

Speaking at the end of the day, Clarke denied that the century - coming after a run of poor scores - had been a relief to him.

"Probably it was more a relief to you guys (the media)," he told Grandstand.

"It was obviously a tough innings - if it was one of my best, you can ask me at the end of the game.

"If we win I'll say it was, but otherwise it (the innings) is useless right now."
Audio: Interview: Michael Clarke (ABC News)

Once Clarke reached his century, it released the shackles, and Clarke and Steve Smith moved the score quickly on.

Smith was finally out for 84 when he chopped on from a delivery from part-timer Dean Elgar with a century beckoning. This brought to an end a 184-run partnership with Clarke.

The score was 4 for 401, and the new batsman Shane Watson was immediately under pressure - he scored a single off his first ball, but it was referred upstairs for a possible LBW before being denied.

South Africa missed a chance to remove the captain when Elgar got one to straighten and hit Clarke on the pad. They failed to review it and replays showed it would have been out.

The pair took the score on to 4 for 434 at lunch, as Australia tightened its hold on the match.

There was a sense as play resumed that the tourists were looking for quick runs.
Duminy leads the way on tough day for South Africa

Watson took 10 runs off the first over from JP Duminy and he hit several big sixes at the start of the session before he went for one big hit too many and was caught by Hashim Amla at long-off off Duminy to depart for 40.

Haddin came in, and the score kept moving in the next few overs as Clarke pulled Abbott to the boundary to make his 150.

Haddin made 13 before he became Duminy's third victim, hitting to Amla at mid-off to make it 4 for 486.

Mitchell Johnson followed first ball, after South Africa reviewed a wide ball down leg which was given out caught behind by the third umpire.

Ryan Harris joined Clarke in the middle, but soon afterwards the rain - which had been threatening for most of the day - finally came down in sufficient proportions to force the players off the field, with Australia 7 for 494.

As the rain continued, an early tea was taken, but after a couple of hours play was called off for the day.

Duminy was the star for South Africa with 4 for 73.

"I think we're in a really good position, 494 on the board," Clarke told Grandstand.

"I thought Dave Warner was outstanding yesterday, Steve Smith played really well again today, the rest of the guys all contributed so we have three very big days ahead of us."

South African paceman Dale Steyn took no part in the day's play after injuring his hamstring on day one. He is expected to face a fitness test before the second innings, if one is required.

Play will start half an hour early - at 10am local time (7pm AEDT) - for days three, four and five to make up for time lost on day two.

Australia could declare overnight and put the Proteas in, or captain Clarke might opt to hit out in the first hour to push the score towards 600.

However the tourists will need time to bowl South Africa out twice if they are to seal a series-clinching victory.